How does the presence of naphthenic acids in crude oil impact refinery corrosion?

Naphthenic acids, also known as cycloalkane carboxylic acids, are organic acids present in some crude oils. They cause a specific type of corrosion called naphthenic acid corrosion (NAC), which is a major concern in oil refineries. NAC primarily occurs at high temperatures, typically between 200°C and 400°C (392°F and 752°F), in the absence of sulfur. The corrosion mechanism involves the reaction of naphthenic acids with the iron in steel, forming iron naphthenates and releasing hydrogen. The iron naphthenates are oil-soluble and are carried away by the flowing oil, continuously exposing fresh metal surfaces to further corrosion. This results in a thinning of the metal walls, particularly in areas with high flow velocity and turbulence, such as elbows, bends, and restrictions in piping. The corrosion rate is influenced by several factors, including the concentration of naphthenic acids in the crude oil, temperature, flow velocity, and the metallurgy of the equipment. High-acidity crudes, with Total Acid Number (TAN) values above 0.5 mg KOH/g, are generally considered more corrosive. The TAN measures the amount of potassium hydroxide (KOH) needed to neutralize the acids in one gram of oil; a higher TAN indicates a greater concentration of naphthenic acids. Steels with higher chromium content, such as stainless steels, are more resistant to NAC due to the formation of a protective chromium oxide layer. However, the high cost of these alloys often limits their widespread use, and strategies like blending high-TAN crudes with low-TAN crudes or injecting neutralizing chemicals are implemented to mitigate NAC.